Electroconductive magnetic carrier, developer using the same and image formation method

ABSTRACT

An electroconductive magnetic carrier which is composed of basic particles, each of the basic particles comprising a binder resin and a magnetic material dispersed in the binder resin, and an electroconductive layer formed on the surface of the basic particle, a developer which is composed of the above electroconductive magnetic carrier and (b) an electrically insulating toner, and an image formation method of forming toner images on a photoconductor by using this developer are described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electroconductive magnetic carrierparticles, a developer comprising the carrier particles and anelectrophotographic image formation method using such a developer foruse with a printer, facsimile apparatus and digital copying apparatus.

2. Discussion of Background

According to the electrophotographic image formation method based on theCarlson process, which is now widely employed, image formation isbasically carried out in such a manner that the surface of aphotoconductor is uniformly charged to a predetermined polarity and thephotoconductor thus charged is selectively exposed to the original lightimages to form latent electrostatic images on the photoconductor. Then,the latent electrostatic images are developed with a developer, so thatvisible toner images can be obtained on the photoconductor. The visibletoner images are then transferred to a sheet of an image-receivingmedium and fixed thereon.

On the other hand, many proposals on the image formation method notusing the Carlson process, but using the rear side exposure system havebeen reported, for example, in The Journal of the Institute of ImageElectronics Engineers of Japan vol. 16, (5), 306 (1987); and JapaneseLaid-Open Patent Applications 61-149968, 63-10071 and 63-214781, bywhich rear side exposure system the image formation apparatus can bemade compact and the image formation process can be made simple.

In the rear side exposure system, the surface of the photoconductor isprovided with a developer to form a developer resident portion, throughwhich the photoconductor is subjected to a cleaning operation, and thephotoconductor is uniformly charged. The light images are applied to thephotoconductor from the rear side thereof and the latent images formedon the surface of the photoconductor are simultaneously developed intotoner images with the developer.

However, there are too many difficult problems in the rear side exposuresystem to put it into practice. More specifically, the requirements foreach function in the rear side exposure system are made extremely severebecause it is necessary to inject the electric charge sufficient for thedevelopment into the photoconductor through the developer accumulated inthe developer resident portion and to form sharp and stable toner imageson the photoconductor by development at a relatively small developerresident portion.

In addition, it is necessary to impart the electroconductivity to thedeveloper since the electric charge is injected into the photoconductorthrough the developer. Therefore, when a developer to be employed is aone-component type developer, an electroconductive magnetic toner isessentially required. The toner image thus formed on the photoconductorcannot be transferred to a sheet of plain paper by the electrostaticimage transfer method such as corona transfer or bias roller transfer.As a result, only a sheet of paper with high resistivity can be used inthis system.

The method of forming a multi-color recording image on a sheet of plainpaper by the rear side exposure system is disclosed in Japanese PatentPublication 60-59592. In this method, however, since a photoconductor isprepared by overlaying an insulating layer on a photoconductive layer,the photoconductor cannot stand the repetition of formation ofmulti-color images thereon. To solve this problem, it is proposed thatthe residual latent image formed on the photoconductor be erased byapplication thereto of a transfer electrical field. This proposal isstill insufficient in practice for obtaining clear images over anextended period of time.

As in the Journal of the Institute of Electrophotography Engineers ofJapan vol. 27, No. 3, p.442 (1988) and Japanese Laid-Open PatentApplication 61-46961, the image formation can be achieved by the rearside exposure and the simultaneous development system, with theapplication of a charging bias and a development bias to aphotoconductor, having counter polarities, using a two-component typedeveloper comprising iron carrier particles with a resistivity of 10⁴ to10⁸ Ω·cm and magnetic toner particles with insulating properties.

However, when the above-mentioned image formation method is applied tothe practically-used copying apparatus, it is difficult to control theimage formation system for obtaining a clear image over an extendedperiod of time, and in addition, the structure of the apparatusnecessarily becomes complicated.

Furthermore, the image formation methods by use of a magnetic carrierprepared by dispersing a magnetic material in a binder resin aredisclosed. For example, a developer comprising the above-mentionedmagnetic carrier and an electrically insulating non-magnetic toner isemployed in Japanese Laid-Open Patent Applications 53-33152 and55-41450; and a developer comprising the above-mentioned magneticcarrier and an electrically insulating magnetic toner is employed inJapanese Laid-Open Patent Applications 53-33152, 53-33633 and 53-35546.In these disclosures, the carrier component in a developer hasinsulating properties and the development is carried out by theconventional Carlson process.

SUMMARY OF THE INVENTION

Accordingly, a first object of the present invention is to provide acarrier having high electroconductivity and a great magnetic force foruse in a developer.

A second object of the present invention is to provide a developer bywhich the rear side exposure system can be satisfactorily carried out.

A third object of the present invention is to provide an image formationmethod using the rear side exposure system, by which method the electriccharge can be readily injected into a photoconductor, a latentelectrostatic image can be satisfactorily developed with a developer,and the obtained toner image can be easily transferred to a sheet of animage-receiving medium.

The first object of the invention can be achieved by anelectroconductive magnetic carrier for use with a developer comprisingbasic particles, each of the basic particles comprising a binder resinand a magnetic material dispersed in the binder resin, and anelectroconductive layer formed on the surface of the basic particle.

The second object of the invention can be achieved by a developercomprising (a) an electroconductive magnetic carrier comprising basicparticles, each of the basic particles comprising a binder resin and amagnetic material dispersed in the binder resin, and anelectroconductive layer formed on the surface of the basic particle, and(b) an electrically insulating toner. The developer thus obtained haselectroconductivity and is capable of producing a toner image withinsulating properties, thereby enabling the toner image to betransferred to a sheet of plain paper by electrostatic transfer withoutdifficulty.

The third object of the present invention can be achieved by an imageformation method of forming a toner image corresponding to a light imageon a photoconductor by use of (a) a photoconductor which comprises alight-transmitting support, and at least a light-transmittingelectroconductive layer and a photoconductive layer which aresuccessively overlaid on the light-transmitting support, (b) a developercomprising an electroconductive magnetic carrier comprising basicparticles, each of the basic particles comprising a binder resin and amagnetic material dispersed in the binder resin, and anelectroconductive layer formed on the surface of the basic particle, andan electrically insulating toner, (c) a development means which isdisposed on the side of the photoconductive layer of the photoconductorand supplies the developer onto the surface of the photoconductor, (d) avoltage application means for applying a voltage across thelight-transmitting electroconductive layer of the photoconductor and thedevelopment means, and (e) an exposure means which is disposed on theside of the light-transmitting support of the photoconductor in such aconfiguration as to be directed toward the development means, comprisingthe steps of:

bringing the developer into contact with the surface of thephotoconductor; and

applying a light image to the photoconductive layer located near aposition where the light-transmitting support and the development meansare mutually directed, from the side of the light-transmitting support,under the application of a voltage across the light-transmittingelectroconductive layer and the development means.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic cross-sectional view of one embodiment of acarrier particle according to the present invention;

FIG. 2 is a schematic cross-sectional view of another embodiment of acarrier particle according to the present invention;

FIG. 3 is a diagram of an image forming apparatus in which the imageformation method of the present invention is carried out;

FIG. 4 is a schematic cross-sectional view of a photoconductor for usewith the image formation method of the present invention;

FIG. 5 is a partially sectional view of a photoconductor and adevelopment means for use in one embodiment of the image formationmethod according to the present invention; and

FIG. 6 is a partially sectional view of a photoconductor and adevelopment means for use in another embodiment of the image formationmethod according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic cross-sectional view of one embodiment of acarrier particle according to the present invention.

In FIG. 1, a carrier particle 11 comprises (i) a basic particle 13comprising a binder resin and magnetic particles 15 dispersed in theabove-mentioned binder resin, and (ii) an electroconductive layercomprising electroconductive finely-divided particles 17 formed on thebasic particle.

In the carrier particle as shown in FIG. 1, it is not always necessaryto coat the overall surface of the basic particle 13 with theelectroconductive layer. Namely, an electroconductive part may be atleast formed on the surface of the basic particle 13 so as to impart thesufficient electroconductivity to the carrier particle. As shown in FIG.1, therefore, the surface of the basic particle 13 may be partiallyexposed without the electroconductive layer. In addition, part of themagnetic particles 15 may protrude over the surface of the basicparticle 13.

Examples of the binder resin contained in the basic particle 13 of thecarrier particle 11 according to the present invention are vinyl resinssuch as polystyrene resin, polyester resins, nylon resins and polyolefinresins.

As the magnetic particle 15 for use in the basic particle 13 of thecarrier particle, a spinel ferrite such as magnetite orgamma-iron-oxide; a spinel ferrite comprising at least one metal, exceptiron, such as Mn, Ni, Mg or Cu; a magnetoplumbite-type ferrite such asbarium ferrite; and finely-divided particles of iron or alloys thereofhaving a surface oxidized layer can be employed in the presentinvention. The shape of the magnetic particle 15 may be a granule, asphere or a needle.

In the case where the carrier particle of the present invention isrequired to be highly magnetized, finely-divided particles of a stronglymagnetic substance such as iron may be employed. It is preferable thatfinely-divided particles of the strongly magnetic substance, that is,the above-mentioned spinel ferrite such as magnetite orgamma-iron-oxide, and magnetoplumbite-type ferrite such as bariumferrite be used as the magnetic particle 15 for use in the basicparticle 13, with the chemical stability taken into consideration.

The basic particle 13 having a desired magnetic force can be obtained byappropriately selecting the kind of finely-divided particles of thestrongly magnetic substance serving as the magnetic particle 15 and thecontent thereof in the basic particle 13. It is proper that the amountof the magnetic particle 15 be 70 to 90 wt. % of the total weight of thebasic particle 13 in the carrier particle 11 of the present invention.

Examples of the electroconductive finely-divided particles 17 for use inthe electroconductive layer of the carrier particle 11 include particlesof carbon black, tin oxide, electroconductive titanium oxide which issurface-treated to be electroconductive by coating an electroconductivematerial on titanium oxide, and silicon carbide. It is desirable thatthe electroconductive materials not losing its electroconductivity byoxidation in the air be used as the electroconductive finely-dividedparticles 17.

It is preferable that the average particle diameter of theelectroconductive finely-divided particle 17 for use in the carrierparticle 11 be 0.5 μm or less, more preferably in the range of 0.01 to0.2 μm.

To deposit the electroconductive finely-divided particles 17 to thesurface of the basic particle 13, for example, the basic particles 13and the electroconductive finely-divided particles 17 are uniformlymixed in such a fashion that the electroconductive finely-dividedparticles 17 may adhere to the surface of the basic particle 13.Subsequently, these electroconductive particles 17 are fixed to thebasic particle 13 with the application of mechanical or thermal shockthereto, so as not to completely embed the electroconductive particles17 into the basic particle 13, but to allow part of theelectroconductive particles 17 to protrude over the basic particle 13.

In the present invention, since the electroconductive layer is coated onthe basic particle 13 by fixing the electroconductive finely-dividedparticles 17 thereto, as shown in FIG. 1, high electroconductivity canbe efficiently imparted to the carrier particle 11. Furthermore, in thepresent invention, it is not necessary to contain the electroconductivefinely-divided particles 17 in the basic particle 13, so that largequantities of the magnetic particles 15 can be contained in the basicparticle 13 to increase the magnetic force of the carrier particle 11.

The apparatus for depositing the electroconductive finely-dividedparticles 17 to the basic particle 13 is commercially available as asurface-modification apparatus or surface-modification system.

For example:

(1) dry-type mechanochemical method

"Mechanomill" (Trademark), made by Okada Seiko Co., Ltd.

"Mechanofusion System" (Trademark), made by Hosokawa Micron Corporation

(2) high-velocity impact method

"Hybridization System" (Trademark), made by Nara Machinery Co., Ltd.

"Kryptron" (Trademark), made by Kawasaki Heavy Industries, Ltd.

(3) wet-method

"Dispercoat" (Trademark), made by Nisshin Flour Milling Co., Ltd.

"Coatmizer" (Trademark), made by Freund Industrial Co., Ltd.

(4) heat-treatment method

"Surfusing" (Trademark), made by Nippon Pneumatic Mfg. Co., Ltd.

(5) others

"Spray dry" (Trademark), made by Ohgawara Kakouki Co., Ltd.

FIG. 2 is a schematic cross-sectional view of another embodiment of thecarrier particle according to the present invention.

A carrier particle 11 as shown in FIG. 2 comprises a basic particle 13which is prepared by uniformly dispersing magnetic particles 15 in abinder resin, and an electroconductive thin film 18 coated on theabove-mentioned basic particle 13.

To form the electroconductive thin film 18 on the basic particle 13, theconventional thin-film forming methods can be employed. For example, athin film of ITO (indium-tin-oxide), indium oxide, tin oxide, aluminum,nickel, chromium or gold may be formed by CVD method, vacuum depositionor sputtering.

It is preferable that the volume resistivity of the carrier 11 of thepresent invention be 10⁵ Ω·cm or less, more preferably 10⁴ Ω·cm or less,and further preferably in the range of 10² to 10⁴ Ω·cm. When the volumeresistivity of the carrier 11 according to the present invention iswithin the above range, the electroconductivity of the carrier is notimpaired and the electric charge can be readily injected into thephotoconductor in such a degree that the charge quantity of thephotoconductor reaches a level sufficient for the rear side exposuresystem. The electroconductivity of the carrier 11 is mainly determinedby the electroconductive layer 18.

To measure the volume resistivity of the carrier 11, 1.5 g of carrierparticles 11 are placed in a Teflon-made cylinder with an inner diameterof 20 mm, having an electrode at the bottom thereof, and the volumeresistivity of the carrier 11 is measured when a counter electrode withan outer diameter of 20 mm is put on the carrier particles, with a loadof 1 kg being applied to the top portion of the carrier particles.

The carrier 11 of the present invention is required to have a greatmagnetic force in some degree. It is preferable that the maximummagnetization (magnetic flux density) of the carrier in a magnetic fieldof 5 kOe be 55 emu/g or more, more preferably 55 to 90 emu/g, andfurther preferably 60 to 85 emu/g. In a magnetic field of 1 kOe, thepreferable maximum magnetization (magnetic flux density) of the carrieris 40 emu/g or more, more preferably 40 to 60 emu/g, and furtherpreferably 45 to 60 emu/g. When the magnetic force of the carrier 11 iswithin the above range, the decrease of the transporting performance ofthe developer can be avoided, and at the same time, the carrier 11 canbe prevented from being attracted to the photoconductor together withthe toner particles.

It is preferable that the average particle diameter of the carrier 11 be5 to 100 μm, more preferably 5 to 50 μm, and further preferably 10 to 40μm. When the particle size of the carrier 11 is within the above range,the surface of the photoconductor can be uniformly charged without anydifficulty and the concentration of toner (the amount ratio of the tonercomponent to the carrier component) can be increased. In addition, thedecrease of the transporting performance of the developer on adevelopment sleeve can be avoided and the photoconductor can be chargedto a predetermined potential.

It is desirable that the true density of the carrier 11 be in the rangeof 3.0 to 4.5 g/cm³. The bulk density of the carrier 11 is preferably2.5 g/cm³ or less, more preferably 2.0 g/cm³ or less, and furtherpreferably 1.5 g/cm³ or less.

The developer according to the present invention comprises theabove-mentioned carrier and a toner component.

As the toner for use in the developer of the present invention, theconventional electrically insulating toner particles with a volumeresistivity of 10¹⁴ Ω·cm or more, preferably 10¹⁵ Ω·cm or more can beemployed. The volume resistivity of the toner can be measured by thesame method as in the case of the carrier.

The toner for use in the present invention may comprise a binder resin,a coloring agent, a charge controlling agent and an off-set preventingagent. In addition, a magnetic toner can be prepared by using a magneticmaterial, which is effective for preventing the scattering of tonerparticles in the image formation apparatus.

As the binder resin for use in the toner, vinyl resins represented by apolystyrene resin, such as styrene-acrylic acid derivative copolymer andpolyester resins are usable.

As the coloring agent for use in the toner, a variety of dyes andpigments such as carbon black can be used.

Examples of the charge controlling agent for use in the toner arequaternary ammonium compounds, nigrosine, bases of nigrosine, crystalviolet and triphenylmethane compounds.

As the off-set preventing agent or image-fixing promoting assistant,olefin waxes such as low molecular weight polypropylene, low molecularweight polyethylene and modified materials of the above compounds can beemployed in the present invention.

As the magnetic material for preparing the magnetic toner, magnetite andferrite can be used.

In the developer of the present invention, it is preferable that theratio of the average diameter of a carrier particle to that of a tonerparticle be 1 to 5, and more preferably 1 to 3. When the ratio of theaverage diameter of the carrier particle to that of the toner particleis within the above range, the surface area of the carrier particlewhich is coated with the toner particles can be controlled to increasethe toner concentration in the developer. Thus, a decrease in imagedensity can be prevented. The average diameter of the toner particle foruse in the present invention is generally 20 μm or less, and preferably15 μm or less.

The charging characteristics of the toner can be adjusted by depositingthe finely-divided chargeable particles to the basic toner particle justlike the carrier as shown in FIG. 1.

The volume resistivity of the developer according to the presentinvention, which can be measured by the same method as in the case ofthe carrier, is preferably 10⁶ Ω·cm or less, more preferably 10⁵ Ω·cm orless, further preferably in the range of 10³ to 10⁵ Ω·cm. When thevolume resistivity of the developer is within the above range, thephotoconductor can be charged to a sufficient level.

It is preferable that the toner concentration in the developer accordingto the present invention, that is, the mixing ratio of toner to thecarrier, be 10 wt. % or more, more preferably 20 wt. % or more, furtherpreferably in the range of 20 to 50 wt. %. When the toner concentrationin the developer is within the above range, the sufficient image densitycan be obtained when the developer is applied to the image formationmethod according to the present invention. In addition, the satisfactorycharge quantity of the photoconductor can be obtained. According to theimage formation method of the present invention, the obtained imagedensity is kept almost constant in a wide range of the tonerconcentration in the developer, so that it is not necessary to controlthe toner concentration in practice or the control thereof can bedrastically omitted.

FIG. 3 is a diagram of an image forming apparatus in which the imageformation method of the present invention is carried out.

In FIG. 3, a drum photoconductor 21 comprises a hollow cylindricallight-transmitting support 23, for example, made of glass, alight-transmitting electroconductive layer 25 formed on the support 23,and a photoconductive layer 27 formed on the electroconductive layer 25.Instead of the drum photoconductor as shown in FIG. 3, a belt-shaped(sheet-shaped) photoconductor is available in the present invention.

Examples of the material for the photoconductive layer 27 includeamorphous silicon (α-silicon), Se-alloys and organic materials. Thematerials of which sensitivity is high and in which the mobility of theelectric charge carrier is high are preferred. With the above pointstaken into consideration, the amorphous-silicon based photoconductivelayer is preferably employed. In particular, a photoconductor preparedby forming at least a light-transmitting electroconductive layer, anamorphous-silicon based photoconductive layer and a carrier-injectionpreventing top layer successively on a light-transmitting support ispreferable.

FIG. 4 is a schematic cross-sectional view of the amorphous siliconbased photoconductor 21 shown in FIG. 3.

The photoconductor 21 shown in FIG. 4 comprises a light-transmittingsupport 23, a light-transmitting electroconductive layer 25 formed onthe support 23, and a photoconductive layer 27 formed on theelectroconductive layer 25. The photoconductive layer 27 consists of afirst carrier-injection preventing layer 26, an amorphous-silicon basedphotoconductive layer 29 and a second carrier-injection preventing layer28.

The first carrier-injection preventing layer 26 can prevent the carrierwith a polarity opposite to that of a development bias from injectinginto the amorphous-silicon based photoconductive layer 29 from thelight-transmitting electroconductive layer 25 when the surface of thephotoconductor 21 is brought into contact with a developer 71 to whichthe development bias voltage is applied. As a result, the noisedisturbing the image formation can be eliminated and the electrostaticcontrast between a light-exposed portion and a non-exposed portion onthe surface of the photoconductor can be increased, thereby improvingthe image quality of the obtained images and decreasing the fogging onthe background of the photoconductor in the course of development.

The first carrier-injection preventing layer 26 is required to have theabove-mentioned electrical characteristics for preventing the carrierfrom injecting from the light-transmitting electroconductive layer 25,and at the same time, to have a high light transmittance, that is, tohave a large optical band gap or high light transmission, not forabsorbing the light for image-exposure which is applied from the insideof the light-transmitting support 23. Furthermore, the firstcarrier-injection preventing layer 26 is also required to have goodadhesion to the light-transmitting electroconductive layer 25 and thephotoconductive layer 29, and have good stability not to undergo thegreat change when the photoconductive layer 29 is formed thereon by theapplication of heat thereto.

Examples of the material for the first carrier-injection preventinglayer 26 for use in the present invention include amorphous silicon suchas amorphous silicon carbide (α-SiC_(x)), amorphous silicon oxide(α-SiO_(x)), amorphous silicon nitride (α-SiN_(x)), α-SiC.O, α-SiC.N,α-SiO.N and α-SiC.O.N; and other organic materials such as polyethyleneterephthalate, Parylene, polytetrafluoroethylene, polyimide,polyfluoroethylenepropylene, urethane resin, epoxy resin, polyesterresin, polycarbonate resin and cellulose acetate.

In the amorphous-silicon based carrier-injection preventing layer, thecontent of an element such as C, N or O can be changed in the thicknessdirection of the layer.

In addition, the p-type or n-type semiconductive amorphous based siliconlayer can be used as the first carrier-injection preventing layer 26. Inthis case, the element of C, O or N may be contained in thecarrier-injection preventing layer 26 in order to increase the opticalband gap and the adhesion to the light-transmitting electroconductivelayer 25 and the photoconductive layer 29, and a doping agent may becontained therein to prevent the carrier from injecting from thelight-transmitting electroconductive layer 25.

It is recommendable that an element belonging to the group IIIa in theperiodic table (hereinafter referred to as the group IIIa element) becontained in the first carrier-injection preventing layer 26 at aconcentration of 1 to 10,000 ppm, more preferably 100 to 5,000 ppm inorder to prevent the negative charge carrier from injecting therein. Toprevent the positive charge carrier from injecting in the firstcarrier-injection preventing layer 26, on the other hand, an elementbelonging to the group Va in the periodic table (hereinafter referred toas the group Va element) may be contained in the first carrier-injectionpreventing layer 26 at a concentration of 5,000 ppm or less, morepreferably in the range from 300 to 3,000 ppm.

When the above-mentioned element is contained in the firstcarrier-injection preventing layer 26, the content of the element may bedistributed with some gradation in the thickness direction of the layer26. In such a case, the average concentration of the element in thelayer 26 may be within the above range.

As previously mentioned, when the group IIIa element is contained in thefirst carrier-injection preventing layer 26, a development bias with apositive polarity is employed; and when the group Va element iscontained in the first carrier-injection preventing layer 26, a biaswith a negative polarity is employed for development.

As the preferable examples of the IIIa group element and the Va groupelement, boron (B) and phosphorus (P) can be respectively used becausethese elements are so excellent in the covalent bonding performance withsilicon (Si) in the layer 26 that the semiconductive characteristics ofthe layer 26 can be sensitively changeable, and the capability ofpreventing the carrier from injecting therein is remarkably improved.

It is preferable that the thickness of the first carrier-injectionpreventing layer 26 be 0.05 to 5 μm, more preferably 0.1 to 3 μm. Whenthe thickness of the first carrier-injection preventing layer 26 iswithin the above range, the injection of the carrier can be readilyprevented and the light for exposure is not absorbed, thereby allowingthe α-silicon based photoconductive layer 29 to efficiently generate thephotocarriers. In addition, the increase in the residual potential canbe prevented.

The α-silicon based photoconductive layer 29 can be formed on the firstcarrier-injection preventing layer 26, for example, by the glowdischarge method, sputtering method, ECR method or vacuum-depositionmethod.

In forming the α-silicon based photoconductive layer 29, it ispreferable that hydrogen (H) or a halogen be contained in an amount of 1to 40 atom. % for treatment of the terminal of the dangling bond.Further, to obtain the predetermined electrical characteristics such asthe electroconductivities in the dark and the light and the desiredoptical band gap, it is recommendable that the group IIIa element, thegroup Va element, and the element of C, N or O be contained in theα-silicon based photoconductive layer 29. As the preferable examples ofthe IIIa group element and the Va group element, boron (B) andphosphorus (P) are respectively preferable because these elements are soexcellent in the covalent bonding performance with silicon (Si) in thephotoconductive layer 29 that the semiconductive characteristics of thelayer 29 can be sensitively changeable, and the improvedphotosensitivity can be obtained.

In particular, when the photoconductive layer 29 comprises α-Si.C, it ispreferable to satisfy the relationship of 0<x ≦0.5 in the composition ofSi_(1-x) C_(x), more preferably, the relationship of 0.05≦x 0.45. Thisis because the resistivity of the thus obtained α-Si.C photoconductivelayer 29 becomes high and the mobility of the carrier is furtherimproved when compared with the α-Si photoconductive layer 29.

Furthermore in the present invention, a function-separating α-Si basedphotoconductive layer 29 consisting of a light-excitation layer capableof mainly generating the photocarriers and a carrier-transport layercapable of transporting the generated carriers is employed, thephotosensitivity, the electrostatic contrast and the dielectric strengthcan be improved.

In forming the light-excitation layer of the above-mentionedfunction-separating type photoconductive layer 29, it is recommendedthat the following points be taken into consideration to permit thelight-excitation layer to efficiently generate the photocarriers:

(1) to decrease the film-forming speed;

(2) to increase the dilution ratio by use of H₂ and He; and

(3) to increase the content of elements which are doped in thelight-excitation layer as compared with that in the carrier-transportlayer.

The carrier-transport layer mainly serves to increase the dielectricstrength of the photoconductor 21 and to smoothly transport the carrierswhich are injected from the light-excitation layer to the surface of thephotoconductor 21. In this carrier-transport layer, the carriers arealso generated from the light transmitted through the light-excitationlayer. Thus, the carrier-transport layer can also contribute to increasethe photosensitivity of the photoconductor 21.

When the above-mentioned function-separating α-Si based photoconductivelayer 29 consisting of the light-excitation layer and thecarrier-transport layer is employed, it is desirable that the thicknessof the light-excitation layer be almost equal to the thickness of alight-absorption layer which can be obtained from the light absorptioncoefficient with respect to the wavelength of light used for theimage-exposure.

The second carrier-injection preventing layer 28 may comprise an organicmaterial or an inorganic material.

Examples of the inorganic material for use in the secondcarrier-injection preventing layer 28 are α-SiC, α-SiN, α-SiO, α-SiC.Oand α-SiN.O. Using such materials, the second carrier-injectionpreventing layer 28 may be formed on the photoconductive layer by theconventional thin film forming methods.

In particular, when the second carrier-injection preventing layer 28comprises 60 -Si.C, it is preferable to satisfy the relationship of0.3<x<1.0 in the composition of Si₁.sbsb.-χ C.sub.χ, more preferably therelationship of 0.5≦x ≦0.95, further preferably the relationship of0.6≦x ≦0.95.

The thickness of the second carrier-injection preventing layer 28 isgenerally in the range of 0.05 to 5 μm, preferably 0.1 to 3 μm, and morepreferably 0.1 to 2 μm.

When the thickness of the second carrier-injection preventing layer 28is within the above range, the image density can be sufficientlyincreased and the dielectric strength of the photoconductor can beimproved, and further, the wear of the photoconductor caused by therepeated operations can be decreased. In addition, even when thedelicate pattern of the electric charge is formed on the photoconductor21, the electrical field, that is, the electric line of force does notspread in the plane direction of the second carrier-injection preventinglayer 28, thereby obtaining the images with sufficient resolution. Inaddition to this, the residual potential of the photoconductor does notelevate because the quantity of the residual electric charge on thesurface of the photoconductor does not increase, so that there is nodecrease in the image density and fogging on the background, andparticularly in the repeated operations, the image density is stabilizedand a ghost image is not caused.

The entire thickness of the photoconductive layer 27 thus obtained,which varies depending on the structure of the above-mentioned layers,is approximately 1 to 20 μm, and preferably 1 to 15 μm, and morepreferably 3 to 10 μm in the case where the LED or EL(electroluminescence) is used as a light source for exposure. When thethickness of the photoconductive layer 27 is within the above range, thelight for exposure can be sufficiently absorbed by the photoconductivelayer 27 to show excellent photosensitivity. In addition, the sufficientdielectric strength of the photoconductor can be ensured, so thatsatisfactory images can be obtained even by application of a low biasvoltage to the photoconductor.

As shown in FIG. 3, an LED array 41, serving as an exposure means (imagesignal exposing apparatus) is disposed inside the light-transmittingsupport 23 of the photoconductor 21 in such a configuration as to bedirected toward a development unit 31, thereby conducting the rear sideexposure through an optical transmitter 43 (Selfoc lens array). Insteadof the LED array serving as the exposure means, an EL light emittingelement array, a plasma light emitting element array, a fluorescent dotarray, a shutter array obtained by combining a light source with liquidcrystal or PLZT (lead (plomb) lanthanum zirconate titanate), and anoptical fiber array can be employed in the present invention.

Around the photoconductor 21, there are situated the development unit31, an image-transfer unit 51 and an image-fixing unit 61.

The development unit 31, which is disposed with facing thephotoconductive layer 27 of the photoconductor 21, serves to supply thesurface of the photoconductor 21 with a developer 71. Anelectroconductive sleeve 35 in the development unit 31 is connected to adevelopment bias source 39 capable of applying a voltage across thelight-transmitting electroconductive layer 25 of the photoconductor 21and the development unit 31. In the development unit 31, a magneticroller 33 having a plurality of magnetic poles (the N and S poles) isincluded in the electroconductive sleeve 35. The magnetic roller 33 maybe fixed to the inside of the sleeve 35 or designed to be freely rotatedtherein.

The thickness of the developer 71 on the sleeve 35 is adjusted by adoctor blade 37. In the preferred embodiment of the present invention,the photoconductor 21 and the electroconductive sleeve 35 arerespectively rotated in the directions of arrows P and S, and thus thedeveloper 71 is transported to the surface of the photoconductor 21.

When the photoconductor 21 and the electroconductive sleeve 35 of thedevelopment unit 31 are driven to rotate in the reverse direction, aspreviously mentioned, a developer is accumulated at a developer residentportion 73 in a gap between the photoconductor 21 and the sleeve 35,which developer resident portion 73 is located lower than the positionwhere the sleeve 35 of the development unit 31 and the photoconductor 21are mutually directed (a point "A" illustrated in FIG. 5) in therotational direction of the photoconductor 21.

The developer resident portion 73 is formed when the developer 71 on thesleeve 35 exceeds the predetermined thickness. The transporting speed ofthe developer 71 from the sleeve 35 to the photoconductor 21, thethickness of the developer 71 formed on the sleeve 35 and the gap widthbetween the sleeve 35 and the photoconductor 21 may be appropriatelydetermined according to the rotational speed of the photoconductor 21and the required size of the developer resident portion 73.

As previously described, the developer resident portion 73 can be formeddownstream the utmost contact point between the development unit 31 andthe photoconductor 21 in the rotational direction of the photoconductor21 when the photoconductor and the sleeve 35 of the development unit 31are rotated in the reverse direction. In this case, the development canbe achieved steadily and the reproduction performance of the obtainedimages becomes excellent as compared with the case where the sleeve 35and the photoconductor 21 are rotated in the same direction, with thecircumferential speed of the sleeve 35 being higher than that of thephotoconductor 21.

The developer 71 for use in the present invention comprises thepreviously mentioned electrically insulating toner and carrier which isprepared by forming an electroconductive layer on a basic particlecomprising a binder resin and a magnetic material dispersed in thebinder resin.

The electroconductive magnetic carrier particles of the presentinvention constitute a magnetic brush, to which toner particles areattached. When the magnetic toner particles are employed, the tonerparticles are caused to attach to the carrier particle mainly by meansof the magnetic force. In the case of the non-magnetic toner particles,they are caused to attach to the carrier particle by means of theelectrostatic charge.

In the image formation procedure, as shown in FIG. 3, the developer 71is transported from the sleeve 35 to the photoconductor 21 andaccumulated at the developer resident portion 73, and the developmentbias voltage with a positive polarity is applied from the developmentbias source 39 to the electroconductive sleeve 35. The positivelychargeable toner is used in this embodiment, but the chargeability ofthe toner and the polarity of the development bias voltage may bedetermined in accordance with the characteristics of the photoconductor21.

When the photoconductive layer 27 of the photoconductor 21 is broughtinto contact with the developer 71, the electric charge from thedevelopment bias source 39 is injected into the photoconductive layer 27through the magnetic brush composed of the carrier particles containedin the developer 71. Thus, the residual electric charge remaining on thephotoconductor 21 caused by the previous image formation process can beerased and the surface of the photoconductor 21 can be uniformlycharged. At the same time, the residual toner particles on thephotoconductor 21, which have failed to be transferred to animage-receiving sheet 81 in the image-transfer unit 51, can be removedfrom the photoconductor 21 by the above-mentioned magnetic brush.

According to the image formation method of the present invention, alight signal corresponding to the original image is applied to thephotoconductor, located near a position where the light-transmittingsupport 23 and the development means 31 are mutually directed, by use ofthe exposure means such as the LED array 41 which is disposed on theside of the light-transmitting support 23 of the photoconductor 21 insuch a configuration as to be directed toward the development unit 31via the photoconductor 21.

When the uniformly charged photoconductor 21 is selectively exposed tothe light signal by use of the LED array, the potential at thelight-exposed portions of the photoconductive layer 27 is rapidlydecreased, thereby generating the potential difference on thephotoconductive layer 27. Depending on the potential difference on thephotoconductive layer 27, the toner particles attached to the magneticbrush are freed from the magnetic force or the electrostatic chargeexerted thereon by the magnetic brush, separated therefrom, and thendeposited to the surface of the photoconductive layer 27.

Even after the photoconductive layer 27 of the photoconductor 21 isseparated from the developer in the developer resident portion 73 whilethe photoconductor 21 is rotated in the direction of the arrow P and thesleeve 35 is rotated in the direction of the arrow S, theabove-mentioned toner particles attached to the photoconductive layer 27remains as they are, so that a toner image 75 can be formed on thesurface of the photoconductor 21. In such a development process, sincethe magnetic brush composed of the magnetic carrier particles is stable,the quantity of the developer in the developer resident portion 73 canbe maintained. As a result, sharp and stable images can be obtained.

In the exposure process by use of the LED array, as previouslymentioned, the light signal corresponding to the original image isapplied to the photoconductor 21, not at the point A illustrated in FIG.5 where the sleeve 35 of the development unit 31 comes to the positionnearest the photoconductor 21, but at the area adjacent to the developerresident portion 73, lower than the point A in the rotational directionof the photoconductor 21. In particular, it is most preferable that thelight signal be applied to the photoconductor 21 at the end point (apoint B illustrated in FIG. 5) where the developer 71 is separated fromthe photoconductor 21.

Since the exposure of the photoconductor 21 to the light signal isconducted at the above-mentioned point in the present invention, theapplication of the development bias voltage to the photoconductor 21 canbe sufficiently stabilized by the time when the exposure process isstarted. Consequently, the surface of the photoconductor 21 can beuniformly charged regardless of the influence of the hysteresis exertedthereon, and the residual toner remaining on the surface of thephotoconductor 21 can be satisfactorily recovered. In addition, sincethe photoconductor 21 is exposed to the light signal to generate thephotocarriers after the application of the development bias voltage tothe photoconductor 21 is sufficiently stabilized, excellent toner images75 can be formed on the photoconductor 21. Since the photoconductor 21is speedily separated from the developer resident portion 73 after theformation of the toner images 75, the toner images 75 on thephotoconductor 21 are not impaired by the mechanical shock such as thecollision or friction between the toner images 75 and the developer 71.Thus, toner images 75 with excellent resolution can be obtained.

Furthermore, the distance between the surface of the photoconductor 21and the surface of the magnetic roller 33 of the development unit 31 islonger at the point B of the photoconductor 21 as illustrated in FIG. 5,adjacent to the developer resident portion 73 as compared with the pointA as illustrated in FIG. 5, where the sleeve 35 of the development unit31 comes to the position nearest the photoconductor 21. Therefore, themagnetic force by which the developer 71 is attracted to the magneticroller 33 is weaker at the point B of the photoconductor 21, so thatpart of the toner images 75 which have already been formed on thesurface of the photoconductor 21 can be prevented from returning to thedevelopment unit 31 by the magnetic force. This can prevent the decreasein image density and resolution of obtained images.

In the image formation method of the present invention, in which thecharging, exposure and development are simultaneously carried out, aspreviously mentioned, it is preferable that the development bias voltagebe as low as 250 V or less, more preferably 10 to 200 V, and furtherpreferably 30 to 150 V. The above-mentioned development bias voltage issufficient to uniformly charge the photoconductor and develop the latentimage into the toner image with the developer. Furthermore, in general,when the size of the carrier particle contained in the developer issmall, the carrier particles are easily attracted to the photoconductortogether with the toner particles in the development procedure. However,the above-mentioned bias voltage is not so high that the carrierparticles can be prevented from being attracted to the photoconductor.

In addition, the previously mentioned amorphous silicon basedphotoconductor is appropriate for the development under applicationthereto of a low bias voltage.

The dynamic resistivity of the developer 71 can be observed by measuringthe electric current flowing to the surface of the photoconductor 21 atthe developer resident portion 73. In the present invention, it ispreferable that the dynamic resistivity of the developer be 10⁷ Ω orless, more preferably 10⁶ Ω or less, and further preferably in the rangeof 10⁴ to 10⁶ Ω.

In FIG. 3, the toner image 75 formed on the photoconductor 21 isattracted by a transfer roller 53 to which a transfer bias voltage witha negative polarity is applied by a transfer bias source 55 andtransferred to the image-receiving sheet 81 in the transfer unit 51.

The toner for use in the present invention has the insulatingproperties, so that the toner image can be steadily transferred to theimage-receiving sheet at high transfer efficiency even though theemployed image-receiving sheet is a sheet of plain paper.

Then, in the image-fixing unit 61, the image-receiving sheet 81 carryingthe toner image thereon is caused to pass through the gap between aheat-application roller 63 and a pressure-application roller 65 to fixthe toner image to the image-receiving sheet 81.

After the image-transfer operation, the residual toner particles on thephotoconductor 21 are removed therefrom in such a manner that the tonerparticles remaining on the photoconductor 21 are attracted to themagnetic brush composed of the carrier particles when the photoconductor21 reaches the position where the photoconductor 21 is directed towardthe development unit 31 and brought into contact with the developer 71.This mechanism necessitates no cleaning member. As a matter of course, acleaning unit may be provided prior to the development unit 31 in thepresent invention.

In addition, a quenching means, for example, a quenching light, capableof erasing the residual charge on the photoconductive layer 27 of thephotoconductor 21 may be provided between the transfer unit 51 and thedevelopment unit 31.

FIG. 6 is a partially sectional view of a photoconductor and adevelopment unit for explaining the another embodiment of the imageformation method according to the present invention.

In this embodiment as shown in FIG. 6, a voltage controllable electrode32 and its attendant members are added to the first embodiment shown inFIGS. 3 to 5.

As shown in FIG. 6, the voltage controllable electrode 32 is provided atthe position where the development unit 31 is directed toward thephotoconductor 21. Preferably, the voltage controllable electrode 32 issituated upstream in the rotational direction of the photoconductor 21with respect to the position where the exposure is conducted by use ofthe LED. In FIG. 6, the voltage controllable electrode 32 is locatedbetween the photoconductor 21 and the sleeve 35, where the sleeve 35 ofthe development unit 31 comes nearest to the photoconductor 21 (at apoint A illustrated in FIG. 6).

The voltage controllable electrode 32 is insulated against the sleeve 35by interposing an insulator 34 between the sleeve 35 and the electrode32. The voltage controllable electrode 32 and the insulator 34 are notattached to the sleeve 35, but independently disposed between the sleeve35 and the photoconductor 21, with constantly maintaining their position(the point A as illustrated in FIG. 6) regardless of the rotation of thesleeve 35 and the photoconductor 21.

Independently of the development bias source 39, the voltage is appliedto the photoconductor 21 and the developer 71 by a voltage controllableelectrode source 36. In addition, to uniformly apply the electric fieldto the photoconductor 21 and the developer 71, it is desirable that thevoltage controllable electrode 32 be a belt-shaped one extending alongthe length direction of the sleeve 35.

The potential of the voltage controllable electrode 32 can bedetermined, independently of the potential of the sleeve 35, bycontrolling the voltage output from the voltage controllable electrodesource 36. For example, the potential of the voltage controllableelectrode 32, which is grounded, may be equal to that of thelight-transmitting electroconductive layer 25 of the photoconductor 21.Alternatively, the potential of the voltage controllable electrode 32may be higher or lower than that of the sleeve 35.

As shown in FIG. 6, when the voltage controllable electrode 32 capableof applying the electric potential to the photoconductor 21 and thedeveloper 71 is provided independently of the application of the voltageby the sleeve 35, the surface potential of the photoconductor 21 ca becontrolled by eliminating the hysteresis of the previous image formationprocesses exerted on the photoconductor 21. Consequently, thedevelopment can be steadily carried out in the case where thephotoconductor 21 is caused to rotate several times to obtain images onone image-receiving sheet 81. Thus, excellent images can be obtained.

By controlling the potential of the voltage controllable electrode 32,the image density of the obtained images can be adjusted and the optimalimage formation conditions without causing the fogging on the backgroundcan be obtained.

According to the image evaluation test which has been conducted by theapplicants of the present application, the reversal development in whichthe toner particles are deposited to the non-exposed portions, and notdeposited to the exposed portions on the photoconductor can beaccomplished by the image formation method of the present invention byincreasing the potential of the voltage controllable electrode 32 anddecreasing the potential of the sleeve 35.

As previously explained, the carrier particles and the developeraccording to the present invention can be adapted to the rear sideexposure system. The carrier particles according to the presentinvention can also be applied to various kinds of image formationmethods which require a developer with high electroconductivity andmagnetic properties. Therefore, the developer according to the presentinvention can also be applied to various kinds of image formationmethods.

According to the present invention, high electroconductivity and strongmagnetic properties can be imparted to the carrier particle. Then, sincethe thus obtained carrier particle is used with electrically insulatingtoner to prepare a developer in the present invention, the developeraccording to the present invention has electroconductivity, and at thesame time, the toner image with insulating properties can be produced byusing such a developer. Therefore, the toner image obtained by thedeveloper of the present invention can be transferred to a sheet ofplain paper by the electrostatic transfer without any difficulty.

According to the image formation method of the present invention, thephotoconductor can be uniformly charged and the latent electrostaticimages formed on the photoconductor can be steadily developed by usingthe developer composed of the carrier which comprises a basic particleprepared by dispersing a magnetic material in a binder resin and anelectroconductive layer formed on the basic particle, and the toner withelectrically insulating properties. Thus, the toner image thus developedby the developer of the present invention can be transferred to animage-receiving sheet by the electrostatic transfer and fixed thereonwith high image density.

Other features of this invention will become apparent in the course ofthe following description of exemplary embodiments which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLE Preparation of Carrier

A mixture of the following components was kneaded and pulverized in ajet-mill, and then classified to obtain a basic particle with an averageparticle diameter of 23 μm for use in a carrier particle.

    ______________________________________                     Parts by Weight    ______________________________________    Styrene/n-butyl acrylate                     25    copolymer (80:20)    Magnetite        75    ______________________________________

100 parts by weight of the above obtained basic particle and 2 parts byweight of electroconductive carbon black particles with an averagediameter of 20 to 30 nm were thoroughly mixed in a Henschel mixer, sothat electroconductive carbon black particles were uniformly attached tothe surface of the basic particle.

Then, the carbon black particles were fixed to the surface of the basicparticle by the application of the mechanical shock thereto using acommercially available surface modification apparatus "HybridizationSystem" (Trademark), made by Nara Machinery Co., Ltd. Thus, the carrierparticle according to the present invention was prepared.

The characteristics of the above-prepared carrier particle were asfollows:

Volume resistivity: 2×10³ Ω·cm

Maximum magnetization (in a magnetic field of 5 kOe): 60 emu/g

Preparation of Toner

A mixture of the following components was kneaded and pulverized in ajet-mill, and then classified to obtain a toner particle with an averageparticle diameter of 7 μm.

    ______________________________________                     Parts by Weight    ______________________________________    Styrene/n-butyl acrylate                     73    copolymer (80:20)    Magnetite        15    Carbon black     5    Polypropylene wax                     5    Charge-controlling agent                     2    ______________________________________

Preparation of Developer

70 parts by weight of the above-prepared carrier particles and 30 partsby weight of the toner particles were uniformly mixed to prepare adeveloper. The volume resistivity of the obtained developer was 3×10⁴Ω·cm and the dynamic resistivity of the developer measured at thedeveloper resident portion was 5×10⁵ Ω.

Formation of Images

The above-prepared developer was supplied to an image formationapparatus, as shown in FIGS. 3 and 5, to carry out the image formation.

In the image formation apparatus, a photoconductor comprising acylindrical glass support with an outer diameter of 30 mm, alight-transmitting electroconductive layer made of ITO formed on thesupport, and an α-silicon based photoconductive layer formed on theelectroconductive layer was employed. The thickness of thephotoconductive layer was 6 μm.

The voltage of +50 V was applied to a sleeve of a development unit by adevelopment bias source. With the application of a transfer bias voltageof -200 V to a transfer roller, the toner images were transferred to asheet of commercially available plain paper in a transfer unit.

When the image formation was carried out at 25° C. and 50% RH, imageswith an image density of 1.4 were steadily obtained.

What is claimed is:
 1. An electroconductive magnetic carrier for usewith a developer, comprising basic particles, each of said basicparticles comprising a binder resin and a magnetic material dispersed insaid binder resin, and an electroconductive layer formed on the surfaceof said basic particle.
 2. The electroconductive magnetic carrier asclaimed in claim 1, wherein said electroconductive layer formed on thesurface of said basic particle comprises numerous electroconductiveparticles which are fixed to the surface of said basic particle.
 3. Theelectroconductive magnetic carrier as claimed in claim 2, wherein saidelectroconductive particles which constitute said electroconductivelayer has an average particle diameter of 0.5 μm or less.
 4. Theelectroconductive magnetic carrier as claimed in claim 1 wherein saidelectroconductive layer is an electroconductive thin film which isformed on the surface of said basic particle.
 5. The electroconductivemagnetic carrier as claimed in claim 1, having a volume resistivity of10⁵ Ω·cm or less.
 6. The electroconductive magnetic carrier as claimedin claim 1, having a volume resistivity of 10² to 10⁴ Ω·cm.
 7. Theelectroconductive magnetic carrier as claimed in claim 1, having amaximum magnetization of 55 emu/g or more in a magnetic field of 5 kOe.8. The electroconductive magnetic carrier as claimed in claim 1, havinga maximum magnetization of 60 to 85 emu/g in a magnetic field of 5 kOe.9. The electroconductive magnetic carrier as claimed in claim 1, whereinsaid magnetic material is contained in an amount of 70 to 90 wt. % insaid basic particle.
 10. The electroconductive magnetic carrier asclaimed in claim 1, having a particle diameter of 5 to 100 μm.
 11. Theelectroconductive magnetic carrier as claimed in claim 1, having a truedensity of 3.0 to 4.5 g/cm³.
 12. An electroconductive magnetic carrierfor use in a developer, comprising basic particles, each of said basicparticles comprising a binder resin and a magnetic material dispersed insaid binder resin, and an electroconductive layer comprisingelectroconductive particles fixed to the surface of said basic particle,said magnetic material being contained in an amount of 70 to 90 wt. % insaid binder resin, and said electroconductive magnetic carrier having avolume resistivity of 10⁵ Ω·cm or less, a maximum magnetization of 55emu/g or more in a magnetic field of 5 kOe, a particle diameter of 5 to100 μm, and a true density of 3.0 to 4.5 g/cm³.
 13. Theelectroconductive magnetic carrier as claimed in claim 12, having avolume resistivity of 10² to 10⁴ Ω·cm.
 14. The electroconductivemagnetic carrier as claimed in claim 12, having a maximum magnetizationof 60 to 85 emu/g in a magnetic field of 5 kOe.
 15. A developercomprising (a) an electroconductive magnetic carrier comprising basicparticles, each of said basic particles comprising a binder resin and amagnetic material dispersed in said binder resin, and anelectroconductive layer formed on the surface of said basic particle,and (b) an electrically insulating toner.
 16. The developer as claimedin claim 15, wherein said toner is a magnetic toner.
 17. The developeras claimed in claim 15, wherein said toner is a non-magnetic toner. 18.The developer as claimed in claim 15, wherein said toner has a volumeresistivity of 10¹⁴ Ω·cm or more.
 19. The developer as claimed in claim15, wherein the ratio of the particle diameter of said electroconductivemagnetic carrier to that of said toner is in the range of 1 to
 5. 20.The developer as claimed in claim 15, having a volume resistivity of 10⁶Ω·cm or less.
 21. The developer as claimed in claim 15, having a volumeresistivity of 10³ to 10⁵ Ω·cm.
 22. The developer as claimed in claim15, wherein the mixing ratio of said toner to said electroconductivemagnetic carrier is 10 wt. % or more.
 23. The developer as claimed inclaim 15, wherein the mixing ratio of said toner to saidelectroconductive magnetic carrier is in the range of 20 to 50 wt. %.24. The developer as claimed in claim 15, wherein said electroconductivelayer formed on the surface of said basic particle comprises numerouselectroconductive particles which are fixed to the surface of said basicparticle.
 25. The developer as claimed in claim 15, wherein said carriercomprising basic particles, each of said basic particles comprising abinder resin and a magnetic material dispersed in said binder resin, andan electroconductive layer comprising electroconductive particles fixedto the surface of said basic particle, said magnetic material beingcontained in an amount of 70 to 90 wt. % in said binder resin, and saidelectroconductive magnetic carrier having a volume resistivity of 10⁵Ω·cm or less, a maximum magnetization of 55 emu/g or more in a magneticfield of 5 kOe, a particle diameter of 5 to 100 μm, and a true densityof 3.0 to 4.5 g/cm³.
 26. A developer comprising (a) an electroconductivemagnetic carrier comprising basic particles, each of said basicparticles comprising a binder resin and a magnetic material dispersed insaid binder resin, and an electroconductive layer formed on the surfaceof said basic particle, and (b) a toner, said toner being anelectrically insulating toner having a volume resistivity of 10¹⁴ Ω·cmor more, the ratio of the particle diameter of said electroconductivemagnetic carrier to that of said toner being in the range of 1 to 5, themixing ratio of said toner to said electroconductive magnetic carrierbeing 10 wt. % or more, and said developer having a volume resistivityof 10⁶ Ω·cm or less.
 27. The developer as claimed in claim 26, having avolume resistivity of 10³ to 10⁵ Ω·cm.
 28. The developer as claimed inclaim 26, wherein the mixing ratio of said toner to saidelectroconductive magnetic carrier is in the range of 20 to 50 wt. %.29. The developer as claimed in claim 26, wherein said electroconductivelayer formed on the surface of said basic particle comprises numerouselectroconductive particles which are fixed to the surface of said basicparticle.
 30. The developer as claimed in claim 26, wherein said carriercomprising basic particles, each of said basic particles comprising abinder resin and a magnetic material dispersed in said binder resin, andan electroconductive layer comprising electroconductive particles fixedto the surface of said basic particle, said magnetic material beingcontained in an amount of 70 to 90 wt. % in said binder resin, and saidelectroconductive magnetic carrier having a volume resistivity of 10⁵Ω·cm or less, a maximum magnetization of 55 emu/g or more in a magneticfield of 5 kOe, a particle diameter of 5 to 100 μm, and a true densityof 3.0 to 4.5 g/cm³.
 31. The electroconductive magnetic carrier asclaimed in claim 1, wherein said electroconductive layer is formed of amaterial selected from the group consisting of carbon black, tin oxide,titanium oxide, indium-tin oxide, indium oxide, silicon carbide,aluminum, nickel, chromium and gold.
 32. The electroconductive magneticcarrier as claimed in claim 12, wherein said electroconductive layer isformed of a material selected from the group consisting of carbon black,tin oxide, titanium oxide, indium-tin oxide, indium oxide, siliconcarbide, aluminum, nickel, chromium and gold.
 33. The developer asclaimed in claim 15, wherein said electroconductive layer is formed of amaterial selected from the group consisting of carbon black, tin oxide,titanium oxide, indium-tin oxide, indium oxide, silicon carbide,aluminum, nickel, chromium and gold.
 34. The developer as claimed inclaim 26, wherein said electroconductive layer is formed of a materialselected from the group consisting of carbon black, tin oxide, titaniumoxide, indium-tin oxide, indium oxide, silicon carbide, aluminum,nickel, chromium and gold.